The actual issue was likely addressed already, but in case you did not figure out the following part, I'd like to say a thing or few about that.

nooster wrote:

I tried putting

Code:

media-video/ffmpeg-1.0.7 libx264

but it gives an 'invalid atom' error

The reason for this is that the format is incorrect, or rather incomplete, and thus rendered invalid (see man portage for more). Whenever you specify a version, you will need to also define if you want only the exact version, the version equal to and greater than... less or greater than... like so:

Code:

=media-video/ffmpeg-1.0.7 x264
>=media-video/ffmpeg-1.0.7 x264

And so on and forth.

Considering it is a package that will likely be used for a while to come, one might omit the version altogether, and define the category/package alone:

Code:

media-video/ffmpeg x264

Or, enable the flag globally. ^^

I hope this helps~_________________Kind Regards,
~ The Noob Unlimited ~